The character has been missing something for a long time and maybe the constant threat of cardiac arrest is precisely that something he's been missing.

No. People care about movie Stark avenging Yinsen and saving those villagers. They care about him saving Pepper. The heart is trivial. In fact, the only moment in the movie where it was cool at all, was as a plot-device to get Stark to reveal that Pepper is the only person he trusts completely when he asks her to help him install his new heart gizmo. There's a reason the movie people eventually disposed of it entirely.

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Take away the conversion experience, replace it with a downward spiral (and no, I don't mean alcoholism, I mean a descent into real depravity) and then provide the last little push into the heart of darkness. Frankly, this would make for two really good "What If" stories if that comic still existed. "What If Tony Stark Went Bad?" - "What If Stephen Strange Went Bad?" - They practically beg to be written!

They've already done this repeatedly, and done it in regular continuity. Civil War, the Tom Taylor run, the whole thing with the universe ending and Stark being outright said to embody death. There already is a character who embodies Stark's worse traits magnified to the point of outright depravity: he's named The Mandarin. I'm sick of Stark stealing the Mandarin's job.